Nanotips have crucial applications in nanotechnology, particularly in scanning probe microscopy and electron microscopy. Therefore, several methods have been developed to fabricate ultra sharp tips [1-5]. These methods can be summarized as: The surface diffusion of atoms by heating the entire tip under an electric field [1,4]; Surface reconstruction and facet formation of pyramidal tip apex by thermal treatment [2]; The deposition of external metal atoms on the tip apex surface [3]; Field assisted nitrogen etching of tungsten tips [5]. All of these methods have exhibited a very sharp end that might terminate with a single atom, as observed in the field ion microscope (FIM). Interestingly, the last atom forms an atomic channel of electrons in the field emission mode which results in a self collimated and coherent electron beam with an outstanding brightness [1,2,6,7]. Nanotips having a single atom tip are ideal for low energy holographic microscopy, and would enable characterization of biological molecules or fragile nano structures with no damage. Nanotips with a well defined shape and atomic scale are crucial for manipulating and characterizing molecules and nano objects in the scanning probe microscope. Furthermore, in a multi-probe scanning probe microscope (SPM), multiple probes are needed to be brought into close vicinity to form physical contacts with a nano object to perform electrical measurements. However, the distance between these probes is limited by the tip size and shape at a mesoscopic scale. These tips are usually characterized in the field ion microscope (FIM) and field emission microscope (FEM) where only the apex structure can be imaged and characterized. The analytical and finite element analyses have shown that, although these tips have a similar apex, they are different and are not really sharp at a mesoscopic scale [8].
Other problems inherent in the previous methods for nanotip fabrication include:                The treatments of these methods are restricted to the very end of the tip, thus resulting in nanotips with a quite small aspect ratio.        These methods, except the nitrogen etching method disclosed in U.S. Pat. No. 7,431,856 B2, only apply to certain metal tips with particular crystal orientation, e.g., W(111) or Ir(121).        All of these methods, except the nitrogen etching method, are blind methods, since the process is unmonitored, and this can result in very poor control of the shape of the produced nanotip.        The nitrogen etching method of U.S. Pat. No. 7,431,856 B2 doesn't work with reactive gases that have an ionization field in the range or less than the evaporation field of the metal; for example, nitrogen etching doesn't work with Ir metal tips.        The nitrogen etching method produces nanotips that are contaminated with the etchant gas species and other contaminants that may accompany the dosed nitrogen gas.        
Due to these various problems, a need remains for improved methods of forming nanotips having controllably very sharp, clean nanotips that terminate with an apex of atomic scale or a single atom and apply to a wide range of metals and semiconductors.